The above discussion worries me a bit. Not because some of the statements made are wrong, but because of the danger of their being misinterpreted.
I hope that no one takes away from this discussion the thought that it is always OK to charge your batteries before leaving them to themselves for the winter. While that might be a viable strategy in the cold frozen north, it is most definitely NOT a good idea for those habiting more temperate latitudes, i.e., those in which the temperatures during "winter" vary widely and/or stay relatively warm.
Keep in mind that all batteries have a self-discharge rate. Flooded batteries lose more each day/week/month from self-discharge than do, e.g., AGMs or gels, and this rate depends both on temperature and on age/condition of the battery.
If the ambient temp is very low, like near freezing, the self-discharge rate will be quite low. This is the condition which I believe MaineSail was referring to.
However, in areas like the Chesapeake daily temps during winter can vary widely....we even get some 70-degree days during December and January....and we can get some very cold (under zero degrees F) days and nights. If your boat is unheated the ambient temp will be affected by the outside temp and the amount of sunlight, as well as the degree of insulation.
What's the danger of leaving a battery less than fully charged? It can and will sulfate, and it will suffer from stratification (i.e., differing electrolyte concentration at various levels). These are the big killers of batteries which, unchecked, will result in loss of capacity and reduced overall life.
Remember, too, that even at a float voltage of 13.2-13.6, a flooded battery will sulfate and stratify somewhat. To avoid these conditions it is necessary to periodically increase the charge voltage to 14.4-14.8 or so.
If you're living in southern latitudes, say South Carolina to Florida or the Caribbean, don't even think about leaving your flooded batteries without at least some means to charge them periodically. In these climates, unattended flooded batteries can be killed by neglect in very short order.
Last edited by btrayfors; 12-05-2011 at 02:05 PM.